That's an interesting thought. Let's assume for a minute that every miner in the world but you shut down. Isn't the difficulty level reset every 2 weeks (give or take) based upon the number of blocks solved and hash rate? If absolutely no blocks are solved (or maybe the OP gets lucky and solves 1 at the current level), how does the network react? Isn't the algorithm designed such that blocks should be solved on average every 10 minutes? If suddenly no blocks are being solved, wouldn't the difficulty level be recalculated immediately such that blocks again are solved every 10 minutes or so?
The info you need is here:
https://en.bitcoin.it/wiki/TargetYou should conclude that a significant and permanent decline in hashrate would disrupt the economy until two retargets have occurred between 2016 and up to 4032 blocks must be found at the new hashrate to achieve stasis. The more blocks that need finding, the less important that second retarget is, in terms of achieving full stasis.
Thus, any highly significant permanent change in hashrate, such that the economy will be disrupted (estimating that is another question I suppose) would result in a need for a fork to permit a manual retarget. Not really that big a deal, in practice, unless the cause of the hash speed decrease was unknown to the community, which seems unlikely.