Eyeball everything and see which pully is out of alignment, and check your idlers to see if the bearings haven't seized (I've had that happen.)
If it's an ilder pully bearing, you can usually get a new one locally, and all you need to change it is a shop vice. If it's a pully out of alignment, it's usually the harmonic damper (big honkin' pully bottom front,) which wants repalcing - which can be DIY in a couple hours or so. Might as well change the front main seal while you're about it - it's just a good idea.
Parts? Whatever a new belt goes for (about $25-30, around here,) figure $5 or so for the ilder bearing, or ~$50 for a new HD. You'll want a HD puller if you don't have one (~$20,) and you can install the new one with a 1/2"-20 bolt and a stack of washers (~$5 - you'll need bolts in a couple different lengths.) If you do a search, I've covered the HD replacement in a bit more detail - so check it out.
It's also worth checking the pully on the front of your air conditioning compressor - it may have frozen as well. Haven't had to replace that one yet, so no good advice from me. I'm sure someone else here has, tho - and I think I've seen it covered in the last fortnight or so...
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