Be it Balance Sheet / Trial Balance not balance, the very first step to approach such issue is

1. Check the Financial Year settings
    - go to Accounting -> Configurations -> Edit Financial Year
    - take note of the Financial Years created
    - make sure that the Financial Year is created up to date

2. Check for imbalance Jtxn
    - go to Accounting -> Error Checking
    - if there is imbalance Jtxn, need to ensure all these Jtxn are rectified

3. Perform Month-End-Processing
    - to do this, go to Accounting -> Month End Processing
    - make sure this Month End Processing page no longer have HIGHLIGHTED rows

4. If the Month-End-Processing still has got HIGHLIGHTED rows after processing
    - check if there are Jtxn dated out of the range of the available Financial Years
    - to do so, go to Accounting -> Journal Listing
    - generate the report From : 0001-01-01 To : one day before the first configured Financial Year
    - then generate again From : one day after the last configured Financial Year, To : 9999-12-30

Once you pass the above initial checking, now, read on base on your scenario

(A) IF TRIAL BALANCE IS NOT BALANCE

1. Check if the reported discrepancy in the Trial Balance is caused by Retained Earnings not being brought forward
    - generate the Trial Balance for the reported Year
    - take note of the discrepancy amount
    - generate the Profit & Loss for the previous year
    - compare the discrepancy amount in Trial Balance against the Profit & Loss amount
    - if they match, then do a manual Jtxn, date it to the last day of the previous year
                            Dr      Cr
        profitAndLoss       XX
        retainedEarnings            XX
    - now generate the Trial Balance again, it should be tally


(B) IF BALANCE SHEET NOT BALANCE
    - make sure you generate the Balance Sheet as of the VERY LAST DAY OF THE MONTH