Others told me I could see the balances, but I don't know where I can see these old adresses. (I only have a password written down on a text file and the .dat file, all other info is lost).
Would've worked only if you are absolutely certain where all your coins are which is nearly always not the case.To go to cmd in bitcoin core, I just need to go to settings I presume? Or do you mean command on a linux device? (I'm using windows as you might have guessed 
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thanks for all the help so far
Command prompt allows you to run Bitcoin Core with a specific flag. If you don't have a basic knowledge of using command prompt, I would encourage you to use this method instead, courtesy of achow101:.thanks for all the help so far
https://achow101.com/2016/07/Bitcoin-Core-Troubleshooting#option-startup
Add -upgradewallet to the string at the end.