Database [databasename] is causing an excessive resource usage problem (cpu or self blocking), which is negatively affecting other customer databases hosted on the same sql server. how to solve?

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SQL-Server

stored-procedure

As per My Hosting Service said, The code with the problem is below:

SELECT
 [dbo].[Student].[StudentID],
 [dbo].[Student].[StudentName],
 [dbo].[Student].[EnrollmentNo],
 [dbo].[Student].[Address],
 [dbo].[Student].[PCT],
 [dbo].[Student].[BranchCode],
 [dbo].[Branch].[BranchName],
 [dbo].[College].[CollegeName]

FROM [dbo].[Student]
LEFT OUTER JOIN [dbo].[College]
ON [dbo].[College].[CollegeID] = [dbo].[Student].[CollegeID]
LEFT OUTER JOIN [dbo].[Branch]
ON [dbo].[Branch].[BranchID] = [dbo].[Student].[BranchID]
WHERE [dbo].[Student].[EnrollmentNo] = @EnrollmentNo

But I cant understand what the problem is.(is any problem with writing [dbo]. before fields?)

They exactly Said is:
It has come to our attention that your database [DatabaseName] is causing an excessive resource usage problem (CPU or SELF BLOCKING), which is negatively affecting other customer databases hosted on the same SQL server.
Please take steps to optimize your database queries or the database itself via indexing, defragmentation, etc. to increase performance and lessen the impact on the SQL server.
The code with the problem is shown..

What I have tried:

Various Stored Procedure optimization Techniques from Internet.