All manually entered transactions for March 2021 are gone, including issued checks.

Let's make sure that you'll be able to retrieve your March 2021 transactions and I'm here to guide you how, xclf07k.

Once transactions are entered in QuickBooks Online (QBO), the data will always remain and shows up unless other users deleted them or made any changes without your knowledge. You can use the Audit Log to check your transactions. To do this, follow the detailed steps below,

1. Click the Gear ⚙ icon and select Audit Log.
2. Select Filter.
3. Use the fields on the Filter panel to choose the appropriate User, Date, or Events filter to narrow the results.
4. Select Apply.
   
   

If you'll be able to confirm that it was deleted, you can undo the transaction. All the details can be seen on the Audit Log under the History columns for the details. I've added a screenshot so you'll be guided visually: 

Once done, you can run reports to double-check the transactions you've entered. You'll have to make sure to filter them to show the information of your business. Here's an article for your guide:  Customize Reports In QuickBooks Online. 

Keep in touch if you need anything else about your transactions. I'll always be right here to get back to you.