As this great blog on repairing the SharePoint 2013 On-Prem reset index stuck resolution, states it’s best to reset your index using powershell.
Sometimes when you search service is paused, you can run
$ssa = Get-SPEnterpriseSearchServiceApplication “YOUR Search Service Application name here” Resume-SPEnterpriseSearchServiceApplication $ssa
OR
$ssa = Get-SPEnterpriseSearchServiceApplication “Search Service Application” $ssa.resume()
but, if it is out of sync ( you have the yellow triangle under index component ) you need to go with Kristopher Loranger‘s advice
Here is an extract from his blog on resetting the index, once you’re at that point.
$ssa = Get-SPEnterpriseSearchServiceApplication "Your Search service application name here"
$disableAlerts = $true
$ignoreUnreachableServer = $true
$ssa.reset($disableAlerts, $ignoreUnreachableServer)
if (-not $?) {
Write-Error "Reset failed"
}