Well I believe I can understand what you mean.
You want to know if a wallet service going offline how you can use your private key ?

As I know the easiest way to import a private key must be using Blockchain.info.
Or you can also use Electrum or Mycelium or even Bitcoin-core. All of these are having options to import your private key (in case of bitcoin-core, you need to import by going into console). Hope the information will help you.