My Samsung S3 keeps on rebooting into recovery, how can I fix this?

[AC Question]

samsung s3 keep on rebooting into recovery

i was trying to install install custom recovery via rom manager that i installed from playstore i opened it and installed recovery the i downloaded a custom rom and i wanted to install that when i boot into recovery the samsung logo shows up and then recovery logo and again it rebooted and shows samsung logo then recovery logo it keep on doing that....
i also tried to open download mod but there my device was unable to connect to pc it doesnot even show that if anything is plugged in i tried different cables but still no luck ... please if Nyone knows the solution tell me...