The title says it all. I installed grub-customizer, made some changes to the GUI, and lost my Windows boot entries. I've tried running sudo update-grub, but it didn't detect Windows. How can I recover the boot option?
You should repair your windows installation with your Windows bootable DVD, using the Repair your computer option, and after you finish with this one, you can download Boot Repair CD iso image and burn it on a regular CD or put it on USB key via Unetbootin. I assume you still have the installation disc for WIN7?
Oh, useless to say that you should get rid of 'grub-customizer' as soon as you can.
Boot Repair CD comes in 2 versions, one is for 32bit OS and the other for 64bit OS. So if you installed Win7 64bit you'll be needing boot-repair-disk-64bit.iso image, and the 32bit iso image for Win7 32bit.
After you make sure that you can boot into Win7, you can place your new Boot Repair CD in tray or use the USB version and restart your computer. Allow the CD to load its system, and choose the Recommended Repair option once you are in the live session with Boot Repair CD.
After you restart your computer again and, hopefully, boot into Ubuntu you should run the sudo update-grub command so Ubuntu be able to list your Windows in the boot menu.
IRST metadata from Windows was causing conflicts after grub settings were updated. I followed the steps in this article to remove it. After that, I ran update-grub and allowed Windows to run chkdsk. All is well, and IRST is reenabled.