vCenter 5.1 to 5.5 upgrades often fail, you need to make a decision, whether to dump your data, in vCenter, otherwise, a call to VMware Support.
You can upload the logs here, to see if there is anything
If you use NetScaler you will want to see these guides. The NetScaler How To Guides show administrators how to get NetScaler up and configured by providing instructions for common scenarios and some not so common ones.
Teach the user how to join ESXi hosts to Active Directory domains
Open vSphere Client: Join ESXi host to AD domain: Verify ESXi computer account in AD: Configure permissions for domain user in ESXi: Test domain user login to ESXi host: