I have a problem in logging into exchange server

I have a problem in logging into exchange server as it fails to authenticate me.
I tried several times to connect through various computers after making profile in each, but still it doesn’t get connected.
Even the third party software that was used in connecting the mailbox now is now unable to connect to the server.
I have also tried changing password, but that too didn’t work.
Everything was normally working even two weeks after I migrated to 2010, but suddenly this problem has begun.
Please help me with new methods/approaches that I could try to correct this problem.
