The Airport Network is not finding a Time Capsule Disk.
The base station is misconfigured, or corrupt.
Reset the Base Station, Softly:
Reset the Base Station, Brutally.
Reconfigure the darn thing.
1.) To reset the Base Station [either way], follow Apple’s instructions Here. [<- Click]
A good article describing the location of the reset button for your model, how to perform a soft or hard reset, and what the ramifications of the said reset would be.
2.) Assuming the above didn’t work; Please deposit twenty-five cents for the next half-hour.
IF; The base station is the only wireless access point:
The Airport Utility located in /Applications/Utilities may ask you to restore your previous base station settings. Click ‘Restore’ to automatically re-enter those settings; after the base station reboots, try the wireless connection again. If all else fails, reconfigure the base station from scratch.
IF; The base station is part of a network (multiple router / gateways):
Consider these potential issues;
• A newer “dual-band” base station has two ‘networks', one for older [WiFi B, and WiFi G (faster) access], and a separate one for newer [WiFi N (even faster, and with greater distance potential) access].
If older (non-N) devices aren’t a consideration, then the router / gateway placement isn’t a problem. However, if older devices which can only connect to older (B, or G) networks are present, they can slow the entire network down to B or G speeds.
• The more networks present, especially for B and G networks [neighbours, and possibly other devices such as wireless phones, or microwaves], the greater the chance of conflict [11 channels or frequencies available; N networks have 149], resulting in slower speeds, and / or connectivity issues.
• The environment plays a part in the distance of a network; distances are quoted as "line-of-sight" [unobstructed]; variations such as buildings [their construction; vent work location, steel surroundings, wiring placement], and airwave congestion may restrict the access to a network.
• Current Comcast / Xfinity ‘Dory’ gateways [SMCD3GNV] cannot have their wireless network deactivated and, as such, always occupy a frequency.
That being read, you may need to consider the placement of the routers / gateways to compliment one another without conflict.
Do you wish to extend the network, like a Wireless Distribution System (WDS)? Several links to help configuring exist for different configurations:
[with Thanks to ddavel]
Airport Utility Help
Designing Airport Networks (Manual)
Various other issues examine here.