Just go back to the date they were last in agreement and go through carefully item by item on the bank statement, making sure each one appears on the QB account, then do the reverse, go through each item in QB making sure it appears on the bank statement.

It's best if a second person can help you, one reading out the items the other checking. 

There *will* be something that has either been missed, mis-enetered, or double-entered. Watch out especially for transposed digits. Believe me it is no good just scanning down by eye to see if everything appears to be there - you have to do it item by item.

the future, it really is worthwhile to reconcile the balance at suitable intervals, typically monthly, but could be more or less frequently depending on the frequency of transactions.

Hope that helps.