My bike suddenly started creaking and squeaking very loud, and moreso the more pressure I put on the pedals. Then the pedals got stickier and harder to turn until I couldn't turn them at all. This was all in the space of 5 minutes. Cleaned the chain as it was a bit gunky, and I can turn the pedals with the bike upside down, but it's quite hard work. The chain's not overly tense anywhere as far as I can tell. The wheels turn fine.
PeteH has implied it, but sounds like something has gone drastically wrong in a bearing somewhere. I'd suggest you remove the chain from the front chainring and try to spin the pedal crank. It should turn easily, with no side-to-side slop.
If not, check the rear wheel for the same, it should not feel gritty or notchy, and should not have sideways movement.
Depends on your mechanical inclination if you want to try fixing this, or give it to a LBS (local bike shop). Related: I'm reasonably competent at bike stuff, but I got them to reinstall my front deraileur properly because after three goes I still couldn't get it right. Took the shop an hour and cost me $52, which was a good trade-off IMO.