No Power to External Hard Drive

MyVolts 12V Power Supply Adaptor Replacement for Iomega ...

https://www.amazon.ca > MyVolts-adaptor-compatible-...
About this item · This 12V high-quality power adapter is compatible with the Iomega LDHD-UP External hard drive. · The plug fits a US 2-pin wall power socket.
Batteries included: ‎No
Connector type: 2-Pin Rating: 4.4 · ‎47 reviews · ‎$29.34 · ‎In stock

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  MyVolts 12V Power Supply Adaptor ... - Amazon.ca

  https://www.amazon.ca > Iomega-Prestige-External-repl...
  This 12V high-quality power adapter is compatible with the Iomega Prestige 1TB External hard drive. The plug fits a US 2-pin wall power socket. This power ...
  Hard Disk Size: ‎1 TB
  Connector type: 2-Pin Rating: 4.2 · ‎12 reviews · ‎$21.73 · ‎In stock

HQRP 12V AC Adapter for Iomega 1TB External Hard Drive ...

https://www.walmart.ca > HQRP-12V-AC-Adapter-for-I...
Buy HQRP 12V AC Adapter for Iomega 1TB External Hard Drive HD Adaptor Power Supply Cord [UL Listed] + Euro Plug Adapter from Walmart Canada.

Iomega External hard drive LDHD-UP Compatible Power ...

https://myvolts.com > product > Iomega_External-hard-...
Iomega External hard drive model LDHD-UP compatible power supply adapter charger.
US$16.49 · ‎In stock

Jim, take he hard drive with you to a electronics shop and let them match it up to a universal power supply/adapter.
Amazon com 12V Iomega LDHD UP External hard drive replacement power supply...

Iomega Prestige 500 GB USB 2 0 Portable External Hard Drive 34169

http://www.aliexpress.com/price/iomega-power-adapter_price.html

NEW AC Adapter For Iomega LDHD UP2 External Hard Drive Charger Power...

https://iomega-na-en.custhelp.com/app/answers/detail/a_id/1235

My Iomega external hard drive was working this morning but now will not open. Thgere is power and seemingly activity but my desktop either does not see it or does not recognise it. it also sounds as though the drive is beeping

Probably not the right answer for your problem. Mine is an external 1 TB Simple Drive. Mine wouldn't respond at all. I unplugged the power cable from the back of the hard drive and replugged it in and instantly it worked (with the computer and hard drive plugged in). I had tried rebooting the computer, plugging/unplugging over and over into the computer and that didn't work.

Some versions of xp like to set permissions excessively, check security settings

Well this is a tough one. I am assumeing that you have done all the normal things.
Most external hard drives are just a standard internal hard drive with a case and a IDE/SATA to USB adapter built in. When you plug in the drive and it powers up listen to the drive if you hear a clunk. Not a the happy ticking of a normal drive but a bad clunk. This can mean that the controller on the hard drive itself is bad.
If everything sounds happy then I would try replacing your power adapter. I know that the blue light goes on but most of that runs off the 5 volt leg of the power supply (I have has some hardware manufacturers actually pull the power light voltage from the USB port Grrr) most of the Large sized external hard drives use a 12 volt adapter and it can have no problem with the step down to 5 volt it may have a problem with the 12v.The 12v is what it uses to spin the drive. Unless you have one of those little drives. They use a laptop hard drive that works of a lower voltage. Most of the time you can get an adapter from Radio Shack. Be sure to bring the old one so they can get the Volts, amps, poleartity and the type of connector. It is worth a try and keep the reciept. If it is just bad I am sorry but the only way to get the data off is to break into the unit and pull the hard drive out. Then use another USB to IDE/SATA adapter to pull the data off.
Hope this helps a little.

if it's 6 months old, you should be still having warranty, suggest you take it back to where you bought it from.

do you have a drive for your external hard drive when you bought it? windows vista is very delicate operating system. some manifacture products can't not be recognized by windows vista.

run the command prompt, by going to start run and entering "cmd" then enter "defrag" followed by the drive letter.  if this doesn't work, enter "defrag /?" with the space for more options