It would help if we knew what agent you’re referring to.
If it’s a regular agent, training up Connections skill will raise your standings and unlock that agent. Their missions are randomly offered from a specific pool of missions for that type of Agent so it may take some time before the same mission is offered again.
If it’s a Career Agent, CCP will usually reset the Agents mission so you can complete it. You will have to file a support ticket:
Type = Game Play Support
Category = Stuck = Stuck Character
If it’s an Event Agent such as a Cosmos Agent or a Data Center Agent, when you decline, quit, fail or let their mission offer expire, that Agent is no longer available.
Hope this helps and good luck to you.