Odd that that would happen out of the blue like that. A quick fix is to open the ever-so-dreaded Terminal. It's a very simple, painless, guaranteed not to cause problems command. Type the following:
<font face = "courier">sudo chmod 775 /Applications</font>
And enter your password when asked. That should give you write access... assuming nothing happened to the group association with that directory. If it doesn't work then post here again. We'll figure it out. There is only one other, painless step, if this one doesn't fix it. (post here anyway, so we know if it did, or didn't work).