I am normally using SQL Server 2012 Management Studio to restore a SQL Server database from a "bak" file. I do this by overwriting an existing database. In the "Options" page, there is a check-box labeled "Close existing connections to destination database", which I mostly check because the target database is always "in use", even if I just restarted the MS SQL Service and I am sure that there are no applications using it.
Anyway, on my client's computer, I saw that that checkbox to close existing connections is grayed out without any information. How and why does this happen? A google search didn't yield any results.
I had the same issue. I simply checked the Checkbox BEFORE choosing the source to restore from. After choosing the source the CB was grey but checked and the restore worked fine.