Frankie,
It is likely your old emails, contacts etc are kept on a server by your provider. For example I have a live.com email account and all my data are on a Microsoft Exchange server.
eM Client creates a copy of your data locally on your PC. When you upgrade from v7 to v8 the format of this database is updated - while v8 can read an old v7 database, v7 cannot read the new format of a v8 database. Unless you keep back-up copies of your data and, in particular, kept a copy of your last v7 version of this database, you’re likely out of luck.
Hence the only way to proceed is to uninstall v8, choose not to save your database if prompted, (optionally) reboot the PC and then install v7 - which you can get from here: https://www.emclient.com/dist/v7.2.40748/setup.msi
If you have a license the PC should remember it. Then - depending on how large your store of old emails is - you wait for some time while v7 copies all your emails from the server.
I hope it goes well; I am back down to v7 and all is working splendidly.