C# IF ELSE statement.

Question

Evening, 

Sorry twice in one night. I have a form that allows new students to add their details and create themselves a password, this is then added to my access database. I added a new student then went to use the username and password credentials to sign into the returning student page. However, a few weeks ago when I wrote the returning students page I wrote an IF ElSE statement, that worked at the time, but now I ask it to sign in with any credentials and the message box that I assigned to the else statement comes up. But (weird part) if I click the okay button to dismiss the message box repeatedly it re-appears, until I click it (about five times) it disappears completely and allows me to sign in. 

Now, I know this is odd and I would post code but I wouldn't know where to start with it. But is there any reason this would happen? It worked fine before I added the new credentials in but I had no need to change the IF ELSE statement and no error messages appear (aside from the one I coded myself into) :D. 

Any help or advice would be great, I cant understand why it's doing it. 
K