Sony Ericsson Xperia X10 mini Network problems!:(

I have had my phone since November 2011 and until April 2012 it was working absolutely fine! However, after April my thoughts changed. First of all, it just started going slow and things took a while to load and some apps wouldn't download or either force closed. A few weeks, maybe months - I can't remember - after, it started crashing more than once a day and failed to load. I deleted some of my apps to see if it would help and also tried to update my phone. Though that action wouldn't let me do so and told me it wasn't available for downloading. I left it then, because I thought it would maybe improve over time. Not long after, it started crashing more often than ever before. At least every 10 minutes, and whilst texting mostly. I left, again, thinking it would be fine. Now, in August/July, it has really knocked my liking for the phone. Despite having my Sim card inserted, my phone tells me there's no network available and provides no signal. I can't text, call, access internet or anything. I can play music and use the camera and the wi-fi only works for 2 minutes at the most. It has only just started doing this these past 2 weeks. I would just like to ask, what might be wrong with my phone and how could I fix it?
Kind regards! (: