Flash it again. Try unchecking auto-reboot and after it's finished flashing just hold your power button until it reboots.
If it still doesn't work, flash the stock rom you are on to get your phone working again. After that you can try again.
ps if you just want to flash a custom rom, you can just flash a custom recovery and skip the rooting part.
Samsung Galaxy S 4 I9505 | WanamLite v3.0
Samsung Galaxy S II I9100 : AOKP 4.2.1
HTC Touch Pro2 | ROM: Valkyrie 6.5.23115TR2.5
HTC HD2 | DarkStone1337 [FroyoStone Sense - V3.2]
HTC Desire : Oxygen v2.1.3