No two ransomware are the same. While some are written quite well and are not decryptable many are just written poorly and flaws have been identified. If you have a sample file or the ransomware note the first step I would take is trying to correct identify the ransomware family and variant. A good starting point is this site which tries to catalog as many variants as possible ID Ransomware
Once you have the ransomware identified it's time to see if there is an available decryptor.
Emsisoft is decent and covers a number of ransomware families, but if that does work and you are able to ID the ransomware you can also check here to see if there is an available decryptor