been faithful about changing filters.

Couple weeks ago, unit outside freezes over, and I realize we have a quite dirty filter. I shut it down, replace the filter and when i start it up several hours later, it's cranking out cold air again. 2 weeks later, it's frozen over again.

I could just repeat the process, but clearly something is not right.

What am I looking at - diagnosis and cost?

(I'm assuming my neglect is the cause and I'll be responsible for the repairs...)