HP pavilion dv 9410us will not start. Worked fine until yesterday, tried to start, lcd lights up for one second and then shuts off. tried removing and switching memory modules.

Try this
1. Remove battery and ac adaptor and then press and hold power button for 10sec and reboot.
2. Reseat ram and hard drive and then check.
3. If u face the same problem nothing much you can do but i can tell you hp has issues with some notebooks motherboard which has a technical fault from factory ship, call them up give the product and serial no: if your computer computer they can tell you if the motherboard can be replaced free of cost or not.
( Before that try going to bios by turning on the computer and start tapping f10 key and if you reach the bios screen run hdd test from the option diagonostic or tools.)