SharePoint Search Index/Partition Degraded

Today I reconfigured my Search Service Application following this post. After restarting the SharePoint Search Host Controller in services.msc, I observed that the search is not working in SharePoint.

Lots of logs, not much sense:

Get-SPEnterpriseSearchStatus tells me that the Index Component and the Partition is degraded:

PS C:\Windows\system32> $ssa = Get-SPEnterpriseSearchServiceApplication
PS C:\Windows\system32> Get-SPEnterpriseSearchStatus -Text  -SearchApplication $ssa
Name      : IndexComponent1
State     : Degraded
State     : List of degraded cells: Cell:IndexComponent1-SPeb9d40f9a806I.0.0;
Partition : 0
Host      : SPSRV

Name      : Cell:IndexComponent1-SPeb9d40f9a806I.0.0
State     : Degraded
State     : (Secondary index cell)
Primary   : False
Partition : 0

Name  : Partition:0
State : Degraded
State :
Degraded cells: Cell:IndexComponent1-SPeb9d40f9a806I.0.0;

Name  : AdminComponent1
State : Active
Host  : SPSRV

Name  : QueryProcessingComponent1
State : Active
Host  : SPSRV

Name  : ContentProcessingComponent1
State : Active
Host  : SPSRV

Name  : AnalyticsProcessingComponent1
State : Active
Host  : SPSRV

Name  : CrawlComponent0
State : Active
Host  : SPSRV 

Solution:

Index reset + full crawl of all my content sources.