What happens if I forget to feed my sourdough starter?
Then feed it again

In my experience, any issues with sourdough, up to and including a surface layer of fungus, formaldehyde smell, and rotten dirty socks smell, can be fixed by simply feeding it. The worse the sourdough's condition is, the longer you have to feed it; if you don't have any of the above mentioned issues, two days consistent feeding should be fine. 

Three notes: 


Usually I put in 2:1 volumetric flour to water.
I have used whole grain wheat (and rye) before, with no difficulties. The consistency will be a bit different, but whole grains can sustain a good long term sourdough. In my experience, a sourdough "trained" exclusively on whole grains does a much better job on any bread recipe with bran in it; whether whole grain or added. 
If you are forgetful about a sourdough, consider putting it in the fridge. I put mine in the fridge and feed it once a week. You can miss a couple weeks, which will then cause the aforementioned smells, but it can be recovered in a few days of consistent feeding on the countertop.