External hard drive can't be detected...y?

Hello,

my 1tb iOmega Prestige External HDD can't be detected by my PC..
I tried taking it off from its enclosure then use a USB-SATA cable but it still persists...
Recovery fees are costly and
i got like a 800+gb's of important data in there.
I recalled that i have scanned it & left it on for about 5hrs.,3 infections were found so i took action & deleted it.After a day i tried but its not accessible anymore.
Can it be just some electronic malfunction due to prolonged use? coz' as far as i'm concerned it's still running smoothly & it doesn't produce no weird sound either.

What should i do?
Pls.help