There are multiple potential reasons why this could happen. Have you checked whether you have enough storage for Bitcoin Core (~70-80 GB is required)? It could also be a memory problem. Anyone that may be able to help you requires more information (e.g. debug.log).
How can I get my funds, or will the payment fail if it isnt synced?
You could extract the private key and import it somewhere else. The payment should not fail, you can check it on any
block explorer by entering the address or TX ID.