unity crashes on start

I tried to upgrade my unity to 5.2.3 it told me that unity couldn’t delete the old unity because part of it was running still. I pressed ignore. It installed but now when I double click unity the window comes up and then vanishes I’ve tried alt which does nothing. I’ve unstalled it and reverted back to the version I had that was working and I’m still not getting unity to start. I double click the icon and it shows the unity screen then it stops . can you help me fix