I have a txt file labelled private key, inside is what i assume to be base64 string ending with =, is there any way I can access the wallet I assume this is the private key for?
Presuming you are in possession of an actual private key and not a random Base64 string, consider this answer to convert it manually yourself or use an external program like this or this to convert it to the Wallet Import Format (WIF).
It would be preferred if you do this in a virtual machine disconnected from the internet for security reasons.
Go to https://www.bitaddress.org/, download the page, scan for malicious code (a good security practice though I can confirm there isn't any), go to Wallet Details*, paste your base64 private key in, go to https://www.blockchain.com/explorer?view=btc, and paste your first public key (uncompressed) and check if there is a balance.
Regardless, then paste your second public key (compressed) and check for a balance.
If you do not see your funds or want to check if there are more funds on SegWit addresses derived from the private key I can give you steps on how to use https://electrum.org, just leave a reply.