can't access phonebook on htc sensation

Can you be a bit more specific about what exactly is happening?
What happens when you go into the People app?

It sounds like there's something wrong with your contacts. I've seen crazy things happen back on Gingerbread where my contacts would get duplicated tons of times and cause plenty of lag in the people app. That could be what's happening here.

Do you have your contacts backed up anywhere?
If you do, go to Settings -> Apps -> All -> People, then clear data.
Then try to open the app and reimport your contacts.

If you don't have them backed up, either try another dialer to see if it works (try Dialer One), or else grab one of the duplicate contact apps on the Play Store to see if it's your contacts that are causing the issue.

Its not really complicated.