@Jazbar-20, Welcome to the HP Support Community!
Are you able to access the internet from other devices?
Have you connected a wireless card in the expansion slot?
According to the product specifications of HP Pavilion Slimline s5503w Desktop PC, it supports only a LAN connection and not Wireless. Click here to check the product specifications.
Try connecting an Ethernet cable to the PC and check if that works. The Ethernet port is at the back of the PC.
Try restarting the PC and the router and check if that helps.
You may refer to this document to troubleshoot network issues.
Let me know how this goes.
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KUMAR0307
I am an HP Employee