Hi Suzanne1968,
Have you been doing a normal reset, or a full 'hard reset'?
If you haven't tried a hard reset, that might be worth trying - but beware this does erase all your content so you would have to reload it again from your Library afterwards. The instruction on how to do this I found on Sony's website - here's a link:
http://support.sony-europe.com/computing/tutorials/prs/bat_rst/ebk_tblsht.aspx?site=odw_en_GB&m=PRS-...
Might also be worth checking that your battery has got a good charge - if you have a frozen screen, the battery charge condition on there might not be up to date if you see what I mean - just another thought.
Hope this helps.
