The entire history of one of my client companies has been deleted. How? How do i correct it?

I'll help you fix that, paul172.

Please know that the Audit Log (history) in QuickBooks can't be deleted. It's possible that the columns of the table were hidden that's why you're unable to see the entire records. To isolate the issue, you'll want to make sure that all the columns are set added and the filter is set to All. Here's how:

1. G to the Gear icon or the Reports menu.
2. Select Audit log.
3. Click the Settings/Gear icon beside the Print icon.
4. Tick all the columns.
5. Click the Filter drop-down menu, then set the User, Date, and Event filters to All.

If the issue persists, I recommend signing in to your QuickBooks account in a private/incognito window. This process will help you rule in/out the browser as the culprit here since this mode doesn't save cache and cookies. Check out the shortcut keys below: 

• Ctrl + Shift + N for Google Chrome
• Ctrl + Shift + P for Mozilla Firefox and Microsoft Edge
• Command+ Shift + N for Safari

If it's now working well, you'll have to go back to your regular browser and then clear its cache to enhance your browsing experience. You may also switch to other supported browsers as an alternative.

Additionally, here's an article that you can read to help track and manage your activities in QuickBooks Online:
 

• Use the audit log

• Utilize the audit log to re-enter deleted transactions

• How clients and accountants view the audit log

Keep me posted if you need more help in dealing with your other tasks. I'll be more than happy to assist you again.