Last year $900 of my money was kept by the irs how do I find out why?

The only way is to contact the IRS.  You can either call directly, or make an appointment at your local office.

 

Individuals    800-829-1040

7 a.m. to 7 p.m. local time

Businesses   800-829-4933

7 a.m. to 7 p.m. local time

From <https://www.irs.gov/help/telephone-assistance>

 

 To find a Local Office:     Local IRS Offices