How do deal with social anxiety?

I’d describe myself as quite a sociable person. I love being out and about – meeting new people.

Lately I’ve been feeling slightly depressed – nothing to warrant too much worry. However, in the last couple of days I’ve avoided doing anything much that involves me leaving the house.
A part of me thinks the best thing to do is get out there & get over it. However, when it comes down to the crunch, I simply haven’t done that. I’ve been avoiding much of any social interaction.
I don’t know what in god’s name is wrong with me and why I can’t get myself out of this funk.
I’ll add that I cannot, or rather prefer not to discuss this with my friends.
Does anyone have any experience of this and how best to handle it?