I am using Sitecore 8.0 update 5, While I try to Identify the contact using
Tracker.Current.Session.Identify(userKey);
I get the error
Sitecore.Analytics.Exceptions.ContactLockException
Adding the whole stack trace below
Exception: Sitecore.Analytics.Exceptions.ContactLockException
Message: Contact 1cd840a6-f367-4b5f-9df1-74240a03fd29 could not be locked in the XDB.
Source: Sitecore.Analytics
at Sitecore.Analytics.Tracking.StandardSession.Identify(String userName)
at Test.Client.Common.Utilities.AnalyticsHelper.MergeContacts(String userKey)
This seems to be an issue with your MongoDB. Check if it's running and if it's reachable to your IIS machine. Also check your log for Mongo erros.
Here is a similar issue: https://community.sitecore.net/developers/f/8/t/1771