Probably, the path to images and stylesheets is wrong or missing stylesheet as a result of an incorrect base URL or improper server rewrites. To solve the problem, first, make sure that your static assets are located correctly (in the frontend and adminhtml directories under <your Magento install dir>/pub/static/). Then check your Apache server rewrites settings and your Magento server base URL. Wrong setting of the AllowOverride directive can prevent the system from retrieving static files.
Also, you can use the magento setup:static-content:deploy command to deploy static view files in the production mode. The command will also work in any mode up to Magento 2.2 version. However, starting from Magento 2.2 version, in case you need to deploy it in the developer mode, add -f key: magento setup:static-content:deploy -f.
