Some backup applications might cause your crawl to pause, or you might be running low on resources and they are paused. Either way, making sure they are resumed at some point is always nice. A few lines to make sure that all Paused content sources are Resumed.
#-- Get the Search Service Application
$searchapp = Get-SPEnterpriseSearchServiceApplication 'Search'
#-- Get the default content sources
$ContentSources = Get-SPEnterpriseSearchCrawlContentSource -SearchApplication $searchapp
#-- Resume Crawl on all content sources Paused
Foreach ($cs in $ContentSources){$cs.ResumeCrawl()}
There are a few other methods to call on the content source object, the link here is for reference.
http://msdn.microsoft.com/en-us/library/aa679491%28v=office.12%29.aspx
Jesper Arnecke 