The SD card only contains Homebrew applications. It really has nothing to do with any modding of your Wii. The homebrew channel, BootMii, will still be on your Wii, as well as any IOS's you might have installed. If you softmodded your Wii, you'll just have to put a USB loader back on the SD card. Then you can run it via the Homebrew channel like before.
Unfortunately, it really sounds like you don't understand much about "modding" the Wii in the first place. It would take me far too long to try to explain what impact your SD card would have on your "modded" Wii, especially without more detailed information about what you are talking about when you say "What do I have to do to be able to play all my games again?". Did you lose games you downloaded (legally) from the Nintendo Store? What "games" are you talking about, and on what media (DVD / USB HD, etc) do they reside?