I just installed a new plastic shut off valve to my hot water supply on my lavatory and I cannot get it to open. I am thinking that the glue may have glued it closed. Any ideas on how to get it unstuck without replacing it?

Shut the water off to the house and try to unscrew the bonnet packing nut. Once the nut's loose see if you can back the stem and washer assembly out counter clockwise. If you can do that then reassemble and test. Good luck, tom

I guess now, you have 20/20 hindsight or you should. Valves are typically installed when they are open.

Plastic shut off valve - you aren't using PVC on hot water are you?