Someone deleted the mpr-file and committed the project. Since this (likely) can not be done in StudioPro, this is probably done from file-explorer using Tortoise. Luckily this is just a branch-line,
Use Tortoise to get the previous commit of the branch line, if you still need it.
I haven't seen this before. But the error seems pretty clear. The MPR file is missing. This is your project package that contains all your code. Not sure if it is possible to commit without having an MPR file, but this is what it looks like to me.
Try opening up your App Directory in Explorer (e.g. C:Program Files/Mendix), find your app branch line and open the folder. The project package (MPR) should be present in the root folder (has mendix logo)
If the MPR is not there it makes sense it doesn't work, because there is nothing to open.
Perhaps re-downloading or even rebranch would work. Also check the line from which you are branching if the MPR is present.