Chapter 1: In a way, it was a peaceful sort of torture
It was a strange peaceful sort of torture in a way: this sensory deprivation tank, but John knew that wouldn't last for long. Soon the psychological aspect of the isolation would take hold. John was starving, and he had a massive gash on his arm which hurt like hell, but considering the circumstance this could be considered a plus point. At least he was receiving some kind of sensory input, all be it a not partially pleasant one, as long as he had something to focus on he could hold off on madness. Luckily he wasn't claustrophobic; at least not yet.
When John had been missing for almost a week Sherlock was frantic. Lestrade had threatened to pull him off the case if he didn't get at least get some rest, and nutrition. Sherlock was sure the man was bluffing, but for once wasn't willing to take even the smallest of risks. Still he could barely face more than a few bites, and sleep was hardly forthcoming.
John's captors had proved most elusive, like whispers on the wind. Trails drifted past just out of focus then scattered too nothing, should Sherlock try and reach out for them. He'd never felt more lost.
Despite a week of nothing eventually a break came, and Sherlock was able to snatch on to a trail and follow it all the way to an abandoned work site. The site was huge, and parts had collapsed revealing a large underground level. John had to be there somewhere.
For now there was nothing to do but search with bare hands. The site had been declared too unsafe for anyone but specialist teams, however even Lestrade did not seem to be paying this much mind. He made only a half hearted attempt to stop Sherlock, before he himself followed him.
Chapter 2: Floating through nothing
The pain and hunger John had at first experienced were long gone faded away to blackness; now there was nothing, even the blackness had receded, he just floated through nothing. At first he'd even enjoyed it, a welcome respite from the madness of his life, but now, now there was neither like nor dislike he simply just was or possibly wasn't; it was hard to tell to any more. His thoughts refused to coalesce into any shape or form; they drifted by like a nebulous cloud tantalisingly close, but remaining just out of reach no mater how hard he grasped.
In the cloud one word stood out clear in the haze: Sherlock. John sounded out the word: 'Sher-lock, Sherlock,' he found he had no idea if he was saying the word out loud or just in his head he briefly wondered if that mattered, but then that thought too drifted away. The word Sherlock remained though like it had anchored itself in John, like perhaps it was special that it meant something. John repeated the word over and over trying to connect it to something, something before the tank. He'd had a life before the tank he knew, he thinks perhaps that it was even a good one, but the nature of that life had left him. It too floated by unreachable in the nebulous cloud.
John realised he was still repeating the word, and he was sure now that it was out loud. He couldn't hear it, but he was sure he could feel his lips moving, and it felt good, so he kept on doing it.
Having something to concentrate on, even it was just the movement of his lips forming one single word, helped to clear up the haze in his head. The swirling nebula began to slowly coalesce, and John found he could once again think clearly.
Fortunately or perhaps more unfortunately (even on a good day John would be pushed to tell which) memory's of 221b, and Sherlock also returned, and John realised that Sherlock (AKA that annoying git) had somehow hijacked his brain and taken over even when nowhere in sight. Worse, John had been repeating his name like it was his salvation, and while John still feverishly hoped Sherlock would come and do some saving (right now would be good) John was no swooning damsel in distress.
John was an army soldier, god dam it, he had his pride, and really salvation was just not a word that should ever be applied to Sherlock Holmes. That monopolising git however did seem very apt. It's not like Sherlock didn't normally monopolise all of Johns time, and most of his thoughts, but this was taking the piss; he wasn't even here, and yet he was all John could think of. John made a vow to himself never to disclose that little tit bit of information particularly to Sherlock; the man was hard enough to live with already.
John was gearing up to thinking about something else, anything else, just to spite Sherlock when it stuck him that while being infuriated over Sherlock he was more coherent he'd been in days. John had enough anger over Sherlock (AKA that annoying git) with his 2am concerts, and body part in the fridge to kept John infuriated but coherent for some time.
If Sherlock was his grip to reality than so be it, but since John was doing all the hard work here, and Sherlock was just mere inspiration, then in no way could Sherlock be thought of as salvation. Period. This was clear even in the haze that still lingered over his thoughts. In fact it was in bold and underlined.