If mining return is less than electricity bill, no miners would like to provide their hash power to support the system, what happens to the system then?
If a miner decides that mining is not profitable (due to electricity costing too much, or whatever reason), and they stop mining, then the whole mining hashpower drops a bit, and the spacing between blocks would get a little slower (by how much depends on how much hash-power that miner that left had). If the spacing remains slow (the miner stays out of the ecosystem), the difficulty level will decrease, and the remaining hashpower of the miners will be able to keep the block timing consistent, but now the rewards are split among less miners, so it's more profitable for the ones who remain, so they continue mining.
In this way the ecosystem is self-stabilizing. If it's too expensive to mine, some drop out, making it profitable again for those left in. If it's way under-cost to mine, lots of miners will join in the game, and the difficulty will go up and the payments less frequent.