missing bill in gst return

GST Department has issued a circular in which it has been clearly mentioned that as GSTR 3B is non-revisable if any mistakes have been committed in a particular month the same can be rectified in later months

Hence if you forgot to take credit on Jan take in February, and make sure you have the original bill and it is genuine.

You can take credit of the bill in Feb.  Time limit of taking ITC is available till either filing return for Sep month following the financial year or up the date of filing annual return whichever is earlier.