upvc door has dropped?

Hi

Just looking for some advice as I am totally lost. My upvc appears to have dropped. I can still open & shut it, although not as well as usual, but have noticed a gap enough for me to get my finger into. I can also feel a draught coming through it.

Sorry for being a bit thick but is it possible to fix it myself or at least get someone to do it for me or will I have to get a new front door. I'm really worried in case it gets worse so any help would be appreciated. The door was fitted about 20 years ago and I have never had a problem with it before.