Checkout page blank and random issues with cart

[kristianer7cd0]

Whenever I add a product to the cart on frontend, I see only the company logo and a blank page on checkout: https://gear.spwn.dk/checkout/

Occasionally certain products cant be added to the cart, depends whether I test it in incognito mode or not. But it changes regularly, sometimes it works, sometimes it doesnt. 

Is this related to this error message I get in the backend: "One or more indexers are invalid. Make sure your Magento cron job is running." ?

I'm a Magento newbie, so please let me know step by step. 
I don't know how to enter commands or look at the folders and files of the server, as the site is being hosted remotely. If you can help me out with that as well, I would greatly appreciate it. 

[Gaurav_Jain02]

Hello @kristianer7cd0 

Any recent extension installation or custom changes?
The message you can see because you might haven't set up the cron in your server. 

If you don't have much knowledge I would suggest hiring any Magento developer to get it fixed.

[Sanjay Jethva]

@kristianer7cd0  
Please disable your custom checkout extension or any extensions related to checkout and switch to Magento's default checkout.