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Your Virtual Machine maybe switched off, see image here. You can turn your VM back on from menu Management>>Boot, see image here.
The CertaSpace Nameservers are ns1.certa.space and ns2.certa.space.
Yes, you have to log into your Domain Registrar's client account and update your nameservers to ns1.certa.space and ns2.certa.space
You can access your Cpanel by visiting your-domain-name.tld/cpanel where tld is either .com, .com.ng, .net, .edu.ng, .co, .org etc.
You can access your webmail by visiting your-domain-name.tld/webmail where tld is either .com, .com.ng, .net, .edu.ng, .co, .org etc.
CertaSpace Cpanel contains both Softaculous and Installatron, so you have access to hundres of Applications/CMS/Blogs. see some Softaculous apps here. And see some Installatron apps here.