Can anyone help? App suddenly says I dont have a company. Been on QB for 5 years. Have uninstalled and reinstalled but now not finding my company?

Thanks for posting your concern here in the Community, @ollyseviour. 

I'm here to share some trouble shooting steps to help find your company. 

Let's reset the data that can resolve common issues when using the mobile app. 

If you're using an Android device, go to the More Options ⋮ menu, then select Settings. Tap Refresh Data, and then choose Yes.

Once done, re-open the app and log back into your account. Then, check if you can already use the app again without any issues.

I've also added this article to learn more about the basics in the QuickBooks Online app: QuickBooks Online app overview.

Let me know if you have other concerns. I'll make sure to get back to your right away. Keep safe always.