i got below problem when changing one database to another database(selecting in drop down ) in ms sql management studio

No, I just checked. It doesn;t appear on my top 100 things-to-do-today list, so "Urgent" it isn't.
It *might* be "very urgent" to you, but it's pretty low priority to me. All you do by stressing your urgency after two minutes is make yourself look like an impatient person, which doesn;t help you get an answer, if we could give one.

Which we can't.

We don't have anywhere near enough information to help you with that: we can't see your screen; we can't access your HDD; we can't read your mind. So we have absolutely no idea what you were doing when the error showed up - so how would you expect us to fix it?

Use the "Improve question" widget to edit your question and provide better information.

Reason for my vote of one: Bumping your question is when you edit it and make no real change in order to keep it near the top of the list. It's rude, it's unnecessary, and (given the content of the question and your "it's urgent" addition after waiting no real time at all) it's abuse.

Don't do it again.