Heater stopped blowing hot air this winter. It was in good shape last winter. Any suggestions for what could be reason or what to check? Thank you

Could be blocked heater core but it appears most likely that the problem is a failed valve shutoff motor that is used to open and close the water by-pass valve from your engine coolant to the heater core. You may be able to repair this yourself. there's a good video of this on Youtube at http://www.youtube.com/watch?v=HywkYSNRGug The valve is only mechanical so not likely it is the problem; more likely the motor that moves the valve control arm. If you have to replace the motor part it is likely $130-150 from the dealer. You may be able to find one at a scrapyard, but they seem to fail regularly so a new one is probably a better idea. Alternatively, you could follow this Youtube video and just rig it up to open and close manually instead of using the motor. That's what I did, disconnected the motor arm and rigged up a little cable running into my glovebox.