Unless you hard coded the bill number into the layout one has nothing to do with the other. The next invoice number is set in Settings > General.

it's two separate problems. I didn't hard code the bill number, and can reset the next number as you suggest.
The more pressing problem is, why did my bill layout change? And how do I restore the old one?

Did the layout change or did the layout assigned to the clients changed? layouts don't change on their own, someone modified it. YOu can change the layout assigned to a client by export from one record to another or by going to bill and statement layouts. Choose the layout you want and click on assign. Then select the clients that should have that layout and click OK.

There are four layouts - Adjustment Note, History Bill Layout, Layout001, and My Bill Layout. I normally use My Bill Layout for all clients. It has changed and I didn't change it.

I see how you can assign a layout to clients, but that's not it. My Bill Layout is not what it used to be.

I did have a problem recently with "not enough memory to load the database" and Timeslips wouldn't load. Our IT guys resolved this by changing the virtual memory on the computer we run Timeslips on. Not sure if that had anything to do with it.

It's almost like the "bill layout" options and the layouts for each have defaulted to the original setting.

You will likely need to create your bill layout from scratch and it is entirely possible that the memory issue you refer to may have contributed to this.

OK - at least I didn't loose the core data! If you think there may be another fix, pls let me know. Cheers, Eveline