Hello,
It sounds that the database is corrupt. I would recommend to create a new database.
You can create a new database by following the steps below.

Shutdown NinjaTrader and Go to the Start Menu
Select My Documents--> NinjaTrader 7--> DB--> NinjaTrader.SDF.
Right click on NinjaTrader.SDF and select "Rename." *Name it "OLDNinjaTrader.SDF."
Then restart the software and NinjaTrader will create a fresh database file to use
Unfortunately the following items stored in the old database will be lost - ATM Strategy templates, Session templates, Instrument Lists / Custom Instruments, historical trade execution data, saved Strategy Analyzer backtest results

To prevent running into situations where you can potentially lose important data, we advise to run regular backups via our inbuild backup feature in NinjaTrader.