my laptop is not entering window start up

After changin my hard drive and usung it for like 1hr or so......i decided to restart the system......but since then the laptop have refused to enter start up page......
I dn't kw if there is sometin wrong with my system....pls kindly help......and if i have 2 get a new hard disk....can yu pls tell me where 2 get origina 1.......cos this is the third hard disk i will be replacing.....am using HP DV6 pavellion.......