No signal to monitor after installing new graphics card

hi,
i recently put together a pc and it was working ok. i installed a new graphics card and this also worked fine for the first 4 bootss. On the 5th boot my montior wouldn't detect the signal from the new card and kept on going into sleep mode. The case lights and keyboard/mouse were on but nothing shown on the screen. Montior gave error that no signal.

my machine spec is below with both old and new cards shown. do you think i need a better power supply or has the graphics developed a fault.
- CPU: Intel Core i7 2600K 3.40GHz 'overclocked' @ 4.50GHz Sandybridge CPU
- Motherboard: Asus P8P67 Intel P67 (Socket 1155) DDR3 Motherboard ** B3 REVISION **
- RAM: corsair XMS3 16GB (4x4GB) DDR3 PC3-12800C9 1600MHz Dual Channel
- Cooler: Corsair A50 CPU Cooler
- Antec Power Supply 750 Watt
- Monitor Dell U2711 - Connected via dual DVI cable
Old card which works fine:
Ge Force Quadro FX 3450/4000 SDI
New card which stopped working
Asus Geforce GTX 560ti DirectCu


any help would be greatly appreciated.

kindest regards

Niem