Hi GMan19,
Was the Mac asleep at 5:15am or awake? Normally WC can reconnect to a WeatherLink IP after sleep, but if there's any problems you're better off quitting WeatherCat before sleeping the Mac (or set it to shutdown at a specified time). Note, that for reliability we suggest not allowing the logger to upload to WeatherLink.com as this can cause it to become unresponsive for long periods of time, or permanently if it fails the DNS lookup for WeatherLink.com.
You can fix the data by first quitting WeatherCat then going to ~/Library/WeatherCatData/Location1/2012 (where ~ is your home directory) then opening the 9_WeatherCatData.cat file with a text editor (something like TextWrangler that won't mangle the file). Scroll to the end of the file and delete the lines back to the time of the last good data - each line in the file looks something like:
12430 t:091510 T:16.80 Ti:21.20 D:12.74 ...
the t: field holds the day, hour and minute, so in this example it's the 9th day at 1510.
Once this is done and WeatherCat has re-downloaded the data from the logger, see the following wiki entry to re-upload the data to Wunderground:
http://wiki.trixology.com/index.php/How_to_force_WeatherCat_to_submit_historical_dataAlternatively, if you have TimeMachine backing up your Mac regularly, quit WeatherCat then restore the WeatherCatData folder from a time prior to the bad data then start it back up again.
Finally, if you open Applications->Utilities->Console.app and have a look through the log around the time you woke the Mac up, are there any entries from WeatherCat that might indicate why it didn't download the data?
Many thanks,
Stu.