If you covered it with Saran wrap moisture could have collected & made it seize. Or it got too hot or wasn't stirred frequently enough.

White chocolate can be very temperamental. It will also seize like that if you overheat it. I have found the method that works for me is to put the broken up chocolate into a glass or china bowl, and microwave on 1/3 power for about 3 minutes, then stir. If it is still in chunks, then give it another 30 seconds on 1/3 power. If it is soft but not runny, continue to stir it as residual heat will generally be enough to finish the melting.
Hope this works for you.

I didnt cover it can that be the problem???

Don't cover it. Moisture will collect on the cover & cause it to seize. If you microwave it, use a lower power setting & stir more frequently.

Don't cover it and also make sure that the bowl you are using is completely dry. It can't contain any liquid or moisture. I also use low heat and check often.

Ann.

I always do mine in a double boiler on low heat.
Ilene

eileen - I've had that happen with it, too. Ginger's right...a couple of drops of condensation or too high power and you can sculpt with it.

Ilene - You have the best idea -- double broiler! I've all but given up trying to do anything in the microwave except melt butter. Just seems to take twice as long to get half the results.

That's why I gave my microwave away in 1995.

I like it for just heating up things but i have never cooked anything in them execpt peanut brittle and it burned and i have never cooked any thing in them again. For heating is ok and to me that is it.
Ilene

I'm with Ilene, break out the double boiler! Wish I had one! I use a flat bottomed pyrex bowl balanced on a pot.

Eileen