What should I do if my CPU is stuck to the cooler?

Sounds like you might be dealing with a sticky situation! First off, if you used thermal adhesive instead of paste, that's usually a bigger problem. Since you’ve tried twisting it and using dental floss, another option is to soak the CPU in isopropyl alcohol (the 99% one is best). Just hang the heatsink over a shallow container with the CPU facing down and pour the alcohol until it covers where they're stuck together. Let it soak overnight and then give the twisting another go. After you manage to separate them, remember to clean off any leftover thermal material before reassembly. Just let everything dry out completely afterward!

Pro tip: putting a CPU in an oven isn’t the best option. That little hint might save your chip! If it’s stuck because the paste went hard over time, heat probably won’t help much. If you didn't get anywhere with twisting, maybe heat from a hairdryer could loosen it up a bit. Just make sure to keep everything unplugged while you do it!

Since you mentioned using the oven, I'm curious why. It's better to use a hairdryer if you have one, just blowing some warm air on the cooler! Just a bit of warm air can loosen things without risking damage.

You might want to try slicing between the CPU and cooler using dental floss or a narrow string! Sometimes, it doesn’t take much to break the bond, and it might just be stuck on a few points rather than the entire surface area.

I've worked in IT for 30 years and honestly, I’ve only replaced thermal paste when I change the CPU. If it's stuck, it might have been glued rather than just pasted on! Just checking, did you apply the paste yourself?