Installing MIAB in old web app

I can’t access old web application after installing MIAB on digital ocean

What do you mean? What old web app? @d8paul

I have a web app on digital ocean and i have just installed MIAB on the same droplet as the web application.
My problem is i cant access the default web app through, all i get is the MIAB static page with " this is a mail-in-a-box"

That would be correct. From the Mail-in-a-Box set up guide:

Can I modify my box after / use my box for something else too? (Advanced.)
No. Mail-in-a-Box must be installed on a fresh machine that will be dedicated to Mail-in-a-Box, and you cannot modify the box after installation (configuration changes will get overwritten by the box’s self-management).

It is clearly stated that Mail-in-a-Box must be installed on a fresh machine. As you have installed it on a droplet with other applications on it you either now have a complete mess or the other applications have been overwritten.

Look and see if your old apps are still in the document root. You may be lucky and they are. If so, my recommendation would be to spin up another droplet for your web application. MiaB is designed to be a stand-alone mail server but it does have the ability to host simple static webpages.
I would seriously consider reinstalling MiaB on a fresh droplet as well … it is hard to tell what may be set up incorrectly due to there already being existing services running on the droplet.

Ok. i still have the files. So i guess i have to run two droplets(machines) for my app and the MIAB

Yes, that is the recommended approach.