The first thing you may want to make sure is event receiver was attached to the list. Following is a powershell script which can help you enumerate the list of event receivers:
$webUrl = Read-Host "Provide web url:"
$listUrl = Read-Host "Provide List url:"
$web = Get-SPWeb $webUrl
$listUrl = $webUrl + $listUrl
$list = $web.GetList($listUrl)
Write-Host "Following are event receivers registered for the list"
foreach ($receiver in $list.EventReceivers)
{
$msg = [String]::Format("Name:{0}, Assembly:{1}, Class:{2}",$receiver.Name, $receiver.Assembly, $receiver.Class);
Write-Host $msg
}
If you see your event receiver listed there, make sure assembly and class names are correct and assembly is deployed.
You may want to keep a watch on ULS logs as you update the item and look for error.
Usually, I write code in event receiver to log verbose messages ULS so that I know what's going on.